This application claims priority under 35 U.S.C. xc2xa7 119 of German application DE 199 28 709.0, filed Jun. 23, 1999.
The invention relates to a hip-joint endoprosthesis system according to the precharacterizing clause of Claim 1.
Hip-joint endoprostheses with a leaflike shaft are disclosed in the patent EP 0 032 165 B1.
In this known construction the shaft is tapered, expanding approximately conically on all sides along its long axis from the distal end over a distance of between about ⅔ and xc2xe of the total length of the shaft, measured along the long axis. In this upper region the medial narrow side of the shaft extends outward from the cone in a continuously curving arch that ends in a collarlike projection. This projection separates a femoral anchoring section of the shaft from a prosthesis neck, which comprises a peg that tapers proximally and serves to receive a spherical joint head. The axis of the prosthesis neck intersects the long axis of the shaft at an angle that corresponds substantially to the angle between the neck and the long axis of the femur in a natural hip joint.
The known construction has proved useful in avoiding a so-called xe2x80x9cclosure rotationxe2x80x9d during insertion of the prosthesis without needing to make the plane of resection at the neck of the femur too deep. This disadvantageous xe2x80x9cclosure rotationxe2x80x9d consists in the following: because the leaf of a conventional prosthesis shaft must have a certain minimal thickness, during complete insertion of such a shaft into the femur the shaft often rotates owing to the multiple curvature of the proximal end of the femur, which causes a straight or even slightly curved object to be diverted away from the wall of the femoral bone.
In clinical practice it has been shown that for large groups of patients who require an artificial hip joint, whereas the overall length of prostheses (to be understood as a projection of the total extent of the prosthesis shaft from its distal end to the outermost end of the peg formed on the prosthesis neck) is substantially the same, because of individual anatomical peculiarities it is necessary both to employ prosthesis shafts with different dimensions and hence also to use different tools to prepare the femur for the implantation thereof. More detailed investigations have revealed that within such patient groups the anatomical differences reside mainly in a difference in the distance separating the point of rotation of the hip-joint ball from the long axis of the femur. The object of the present invention, as follows, derives from this finding.
The object of the invention is to disclose a hip-joint endoprosthesis system of this generic kind that provides a saving of time and cost during implantation, in particular during the work of preparing the bone for implantation of the prosthesis shaft.
This object is achieved by a hip-joint endoprosthesis system with the characteristics given in Claim 1.
The invention includes the essential idea of providing a hip-joint endoprosthesis system the length of which, with reference to the long axis of the prosthesis shaft, is substantially constant. This system has the considerable advantage that when preparing the femur for implantation, it is possible to work with a uniform rasped dimension for all prosthesis sizes within the system.
The invention further includes the idea that this constancy of overall length of the prosthesis shaft, combined with differently long prosthesis necks, can be achieved in one variant by altering the angle between the axis of the prosthesis neck and the long axis of the anchoring section, according to the length of the prosthesis neck. In another variant this angle (the so-called xe2x80x9cCCD anglexe2x80x9d) is kept constant but an xe2x80x9coffsetxe2x80x9d section is inserted into the prosthesis neck in its long direction, such that the amount of lateral offset to be provided is determined by the required length of the prosthesis neck, i.e. by the required distance between the long axis of the anchoring section and that point (the xe2x80x9creference pointxe2x80x9d) in the region of the peg of the prosthesis neck that coincides with the central point or point of rotation of the subsequently attached joint ball. In principle a combination of the two variants is also possible.
The second variant is most appropriate for relatively great prosthesis neck lengths, for which it might be necessary to make the prosthesis neck angle disadvantageously small in order to ensure uniformity of the overall length of the prosthesis shafts within the system. In such cases the advantages obtained with the prosthesis-neck offset section counterbalance the possible disadvantages of the irregularity (a xe2x80x9ckinkxe2x80x9d) in the medial arch line associated with inserting the offset (which in some circumstances can result in highly stressed pressure sites in the corresponding region of the spongiosa). In the range of less great prosthesis neck lengths, however, from the current viewpoint the adjustment of CCD angle in accordance with the prosthesis neck length appears to be the preferred variant.
A further essential aspect consists in constructing the prosthesis neck such that it is flattened in cross section. Its cross-sectional configuration is thus in some degree matched to the leaflike cross section of the femoral anchoring section; the spectrum comprises more or less flat ellipses, rectangles with rounded corner regions, combinations of circular and straight sections, or the like. In particular in combination with above-mentioned variant of a prosthesis shaft having an offset in the long direction of the prosthesis neck, it is advantageous in special embodiments for the cross-sectional configuration to change along the length of the neck. For example, an originally elliptical cross section in the region of the medial arch can become circular proximal to the offset section, or the cross section of the ellipse can change. In another embodiment the neck cross section can be approximately rectangular distal to the axis offset point, whereas proximal to this point it is square or has a different rectangular shape.
In one special embodiment, advantageously matched to the anatomical relationships, the shaft cross section is approximately trapezoidal at least in the proximal section, in particular has a symmetrical trapezoidal shape with two equally long longer sides that in cross section delimit the anterior and posterior surfaces of the shaft, and two differently long shorter sides of which the shorter corresponds to the medial surface and the longer, to the lateral surface of the shaft.
Additional advantageous structural features and alternatives of the construction in accordance with the invention will be apparent from the subordinate claims and are described in detail with reference to the following exemplary embodiments and to the attached drawings, wherein: